Motor control system



Patented June 20, 1944 MOTOR CONTROL SYSTEM David C. Wright, Euclid,Ohio, assignor to The Electric Controller 8; Manufacturing Company,Cleveland, Ohio, a corporation Ohio Application March 30, 1942, SerialNo. 436,780

24 Claims.

This invention relates to a motor control system and more particularlyto a .control system for a direct current hoist motor wherein thearmature winding and field winding are connected in series with eachother for hoisting and in parallel with each other for lowering, andwherein a limit switch is used for automatically setting up an emergencydynamic braking circuit for the motor in event the motor raises the loadtoo high.

Although it obviously has other applications, the invention is describedherein in connection with its application to a hoisting system.

In the operation of a hoist by a direct current motor, it is commonpractice to control the lowering operation 'by causing the motor eitherto drive the load as a shunt motor or to retard the load as a generator,the size of the load determining th action of the motor. Suitableresistors in series with the motor and also in the field and armaturecircuits are arranged to be regulated to control the lowering speed. Them0- tor is generally connected as a series motor for hoisting purposesbecause of the good speedtorque characteristics of such a connection.Suitable resistors in series and in parallel with the motor e ra ged tobe regulated to control the hoisting speed.

It is also common practice in such hoist control systems to providesuitable control means such as a limit switch, or limit stop, whichopcrates to establish an emergenc dynamic braking circuit in response tomovement of the hoist beyond a predetermined point in the hoistingdirection. Operation of the limit switch also disconnects the motorcircuits from the power source which causes the usual series connected,electromagnetically released, friction brake to be applied. In the eventof over-travel in the hoisting direction, therefore, the motor isbrought to rest under the combined influence of dynamic and frictionbraking. In such prior systems, after the limit switch has operated tostop the motor, a lowering circuit may be established by moving themaster switch to a lowering position to enable the hoist to be loweredout of the limit. While the limit switch remains in its operated ortripped position, the motor can be connected only I as a series motorwiththe armature energized at ing circuit is immediately establishedcausing the motor to slow down suddenly. As a result the entire hoistingmechanism is subjected to severe mechanical strains and the motor is aptto have commutation difficulties. The change of speed is especiallygreat if the master switch is in the first lowering position which, if aheavy load is being lowered, is the preferred position, but even if themaster switch is in thlast lowering position, the speed change uponresetting of the limit switch is also very large.

It is therefore the general object of this invention to provide a motorcontrol system which does not have the aforementioned disadvantages butwhich retains all of the advantages of the usual dynamic braking hoistcontrol systems and which eliminates any sudden changes in speed whichwould subject th hoist mechanism and motor to severe shocks and strains.

More specifically, a'principal object of this invention is temporarilyto connect a direct current hoist motor as a. slow speed series machinefor lowering operation after the limit switch has tripped to set up adynamic braking circuit, and to maintain said connection onlyuntil thelimit switch resets due to said lowering action.

A more specific object is to obtain the slow speed series motorconnection by completing an armature shunt circuit while lowering thehoist out of a limit stop which has operated to con nect the motor in adynamic braking circuit.

Another object is to effect lowering of a direct current motor drivenhoist, after the hoist limit switch has operated to connect the motor ina dynamic braking circuit, by connecting the motor to the source ofpower as a series machin with an armature shunt and maintaining saidconnection until the limit switch operates to reconnect the motor to thesource of power as a shunt motor for dynamic lowering.

Many cranes include a trolley structure on which the hoist motor,electromagnetic brake, limit switch and certain other parts of thehoisting mechanism are mounted. The electromagnetic control panel ormanual controller is usually mounted on the crane bridge instead of thetrolley structure which moves along the bridge. This requiresconnections from the panel or controller to the apparatus on the trolleystructure by means of trolley bars and collector shoes. Heretofore, mostof such control systems have required four electrically independenttrolley bars. Trolley bars are expensive to install, take up valuablespace, and are a potential source of trouble. Accordingly, a furtherobject of this invention is for eliminating the aforementionedundesirable operating characteristics without increasing the number oftrolley bars.

In carrying this invention into effect in one form thereoi', normallyopen contacts are arranged to complete, when closed, a shunt circuitaround the armature of a hoist motor. An armature shunt on a seriesconnected motor causes a slow motor speed because part of the currentused in exciting the field is shunted from the armature, thus reducingto below normal the ratio of armature current to field current. Thenormally open contacts are arranged to be closed in response toenergization of a winding connected in the motor circuit so as to beresponsive to the counter voltage of the motor.

As soon as the motor has accelerated to a slow speed as it is loweringout of the limit stop as a series connected machine, the winding isenergized sufficiently to cause closure of the contacts to effectcompletion of the armature shunt circuit and thereby to maintain theslow motor speed until the limit switch resets.

Other objects and advantages will become apparent from the followingspecification wherein reference is made to the drawing, in which Fig. 1is a simplified wiring diagram 01 the control system;

Fig. 2 is a diagram indicating the position or the switches for thevarious master switch positions during a hoisting operation;

Fig. 3 is a diagram indicating the position of the switches for thevarious master switch positions during a lowering operation;

Fig. 4 is a simple diagrammatic sketch of a crane hoist drive and limitswitch. operated thereby;

Fig. 5 is a simplified wiring diagram of a portion of the control systemshowing a modification of the invention; and

Fig. 6 is a simplified wiring diagram of a por tion of the controlsystem showing another modification of the invention.

Referring to Fig. 1, a direct current motor Hi having an armaturewinding ii, a commutating field winding [2, and a series field windingI; is arranged to be connected for energization from direct currentpower supply conductors 20 and 2|. For the hoisting direction ofoperation switches l4 and I! are arranged to connect the armaturewinding ii, winding l2, the series field winding iii, a winding I! foran electromagnetically released brake, and a resistor 18 in series witheach other across the conductors 2|! and 2|. With the switches l4 and I!closed the brake is released and the motor I0 is connected as a seriesmotor and exerts a hoisting torque on a hook 4B suspended from a cable45 wound on a drum 25 as shown in Fig. 4. The drum 25 is arranged to bedriven by the motor l0 through gearing 48.

If the switch [4 is opened and a switch It is closed, a circuit iscompleted for lowering operation from the conductor 20 to the junctionpoint 8 between the series and commutating field windings. The circuitdivides at the point 8. One branch of the divided circuit includes theseries field winding I3 and the other branch includes the commutatingfield winding 12, the armature winding li, a resistor l8, and a normallyclosed switch 22. The divided circuits join at the junction point 9 andthe lowering circuit is completed to the conductor 2| through the brakethe commutating field winding IS, the resistor l6, and the switch [1.

to provide a hoist control system including means With the switches l8and i1 closed the motor I. is connected as a shunt motor across theconductors 20 and 2i and is adapted to drive the hoisting drum 25 in adirection tolower the hook 4G or to retard by dynamic braking action thelowering of an overhauling load attached to the hook 48.

Speed control during both lowering and hoisting is obtained bymanipulation of the switches 22, 26, 21 and 28. The switches 26, 2'! and28 are arranged to shunt respective series connected sections of theresistor [6. The switch 26 when closed short circuits the resistancesection lie, the switch 21 when closed short circuits the resistancesection I62), and the switch 28 when closed short circuits theresistance section l6c. Speed control in the lowering direction is obtained also by manipulation of switches 29 and 30 which completecircuits from adjustable points on the resistor 19 to an adjustablepoint on the resistor l6 and to the terminal of the resistor 16 which isconnected to the switch l1, respectively. The control system so fardescribed is similar to that shown in my reissued Patent No. 19,5 47,reissued April 23, 1935, but this invention is applicable to other typesof dynamic lowering hoist controllers as well.

When the control system is used for hoisting service the switches andresistors so far described are generally mounted on a relativelystationary part of the hoisting mechanism and the necessary electricalconnections are completed through the trolley bars and trolley shoes tothe motor III, brake winding 15, and a limit switch 4|] to be described.The switch 18 and the junction point 8 are connected through a trolleybar 3|, the switch 14 and the armature winding ii are connected througha trolley bar 32, the switch 22 and the junction point 9 are connectedthrough a trolley bar 33, and the brake winding i5 and the resistor iiiare connected through a trolley bar 34.

The hoist limit switch 40 which may be one of a number of well knowntypes is shown diagrammatically in Fig. 4. The switch 40 comprises aweight 41 suspended by a cable 49 from a counter-balanced lever arm 4|which is arranged to actuate a contact arm 42 through a suitable snapaction device, shown as a coil spring 43. Attached to the contact arm 42are the bridging portions of a pair of normally-closed contacts 42a and42b and a pair of normally-open contacts 420 and 42d. A projection 44attached to the cable 45 is arranged to engage the weight 41 when thehoist motor Ill lifts the hook 4B beyond a predetermined point.Engagement of the projection 44 with the weight 41 releases the leverarm 4! and permits the spring 43 to move the contact arm 42 from theleft-hand position shown to a right-hand position. This movement of thecontact arm 42 causes opening of the contacts 42a and 42b and closing ofthe contacts 420 and 42d with a snap action.

Referring again to Fig. 1, opening of the contacts 42a of the limitswitch 40 disconnects the armature winding H from the supply conductor20 and opening of the contacts 42?) interrupts the direct connectionbetween the commutating field winding l2 and the series field winding13. Closure of the contacts 420 and 42d complete a dynamic brakingcircuit for the motor Ill from the left-hand terminal of the armaturewinding i i through the contacts 420, the series field winding Hi fromleft to right, the junction point 9, the contacts 4203, a resistor 50,the junction point 8 and the commutating field winding I2 to theright-hand terminal of the armature II. It is to be noted that thecontacts of the limit switch 40 are all between the motor In and thetrolley bars indicating that the limit switch 40 is mounted with themotor on the moving part of the hoist.

The switches I4, II, I8, and 26 through 30 are preferably normally-opencontacts of electromagnetically operated contactors and the switch 22 ispreferably a normally-closed contact of an electromagnetically operatedcontactor. The contactors may be operated from a multi-position,manually-operated master switch and the operating sequence of some ofthem controlled through suitable relays in a well known manner. Thepreferred sequences for hoisting and lowering operations are shown inFigs. 2 and 3, respectively. The dots indicate which of the switches areclosed in the respective master switch positions.

As shown in Figs. 2 and 3 the switch 22 is closed when the master switchis in its off position. For lowering operation, movement of the masterswitch to the first position in the lowering direction causes closure ofthe switches I'I, I8, 21 and 28 as shown in Fig. 3. The motor III is nowconnected across the power conductors 20 and 2I as a shunt motor withthe resistor section id in series with both the armature winding II andthe field winding I3, and the resistor I9 is in series with the armaturewinding II through the switch 22. With this connection only a smalllowering torque is developed if the load is not overhauling, andoverhauling loads are prevented from lowering rapidly because of dynamicIbraking action.

In the second lowering position the switch 22 opens and the switch 29closes, causing an increase in motor torque or an increase in thedynamic braking resistance and a consequent increase in lowering speed.In the third lowering position the switch 28 opens to increase theresistance in the field circuit and also to increase the dynamic brakingresistance. In the fourth lowering position the switch 21 opens, causinga further increase in the field circuit resistance and a furtherincrease in the dynamic braking resistance. In the fifth loweringposition the switch 29 opens and a switch 30 closes. All of the resistorI6 is now connected in the field circuit and in the dynamic brakingcircuit and but a small portion of the resistor I9 remains in thearmature circuit. It is obvious that movement of the master switch fromthe off position through the lowering positions causes a gradualincrease of motor speed for both overhauling and non-overhauling loads.

If at any time during lowering operations the master switch is returnedto the off position, a

dynamic braking circuit is completed throughthe resistor I9 and theswitch 22 causing retardation of the motor I and concurrently the brakewinding I5 is deenergized which permits the fric tion, the switch 22opens to interrupt the shunted motor connection causing an increase inhoisting speed. The switches 26, 21, and 28 are closed in sequence asthe master switch is moved to the third, fourth, and fifth hoistingpositions respectively, causing gradual acceleration of the motor III toits normal hoisting speed, theultimate speed depending upon the size ofthe load being lifted. Movement of the master switch from any of thehoisting positions to the off position sets up a dynamic braking circuitthrough the resistor I9 and the switch 22 to retard the motor by dynamicbraking action. At the same time the brake winding I5 is deenergized andthe electromagnetic brake consequently is released to assist in stoppingthe motor and to hold the load in its raised position.

If the load is hoisted too high. the projection 44 engages the weight 41and permits the spring 43 to move the contact arm 42 to its right-handposition. Opening of the contacts 42a and 421) as a result of theoperation of the contact arm 42 disconnects the motor I0 and the brakewinding I5 from the source of power supply and closure of the contacts420 and 42d completes the previously traced emergency dynamic brakingcircuit forthe 'motor I0 through the resistor 50. The motor I0 isbrought to a stop by the combined action of the dynamic braking torqueand the friction brake which was applied when the winding I5 wasdisconnected. The usual limit stop permits the crane hook to travel ashort distance after the weight has been lifted, and the time for thedynamic braking and friction braking to become effective permits thehook to travel a short additional distance before it is stopped. Thisovertravel might b ten or twelve inches. To reset the limit switch insuch case requires seven or eight inches of downward travel of the cranehook. If there is considerable gearing such as shown at 48 between thedrum 25 and the motor III, this distance of downward travel will equalabout 50 revolutions of the motor which is sufiicient to permit themotor III to accelerate to a rather high speed when connected as aseries motor.

Movement of the master switch to th first lowering position with thelimit switch 40 in the tripped position completes a circuit through theswitch I8, the junction point 8, the commutating field winding I2, thearmature winding II, the contacts 420, the field winding I3, from leftto right, the junction point 9, the brake winding I5, the switches 28and 21, the resistor section H541, and the switch H to the conductor 2|.

The motor III is thus connected as a series motor across the powerconductors 20 and 2| through the resistor section Ilia. This connectionresults in a large downward torque which causes the motor III toaccelerate rapidly so that it has reached considerable speed before thelimit switch 40 trips. If a large overhauling load is attached to thehook 46, the speed is of course still greater. With this connection themotor III is shunted through the resistor 50 and the contacts 42d, butthis motor shunt has little effect in reducing the motor speed due tothe low resistance of the resistor section I6a.

When the limit switch 40 returns to its normal position with the masterswitch in the first or subsequent lowering positions, the motor I0 isreconnected as a shunt motor. As previously mentioned, the shunt motorconnection for the first point lowering is a slow speed point. Thiscauses a sudden slowing down of the motor III upon resetting of thelimit switch "I as the hoist moves out of the limit. This slowing downis so severe in some cases as to cause damaging mechanical strains inthe hoisting mechanism and also interferes with proper commutation ofthe motor [0. If the master switch is in other lowering positions thechange in speed as the limit switch resets is also such as to causeserious damage.

To eliminate this operating defect in accordance with this invention, anelectromagnetic switch 5| having an operating winding 5| w andnormally-open contacts 5hr is utilized. The winding Blw is connectedacross the motor Ill from the junction point I through the winding H10and the contacts "d to the junction point 0, and the contacts ila arearranged to complete a circuit around the armature winding 1 I and thecommutating field winding I2 through a resistor 52. When the limitswitch 40 is in its tripped position and the master switch is moved to alowering position, the counter electromotive force of the motor I isimpressed on the winding Slw. Due to the rapid acceleration of the motorIll under these conditions, the counter voltage rises rapidly. Thewinding SM is arranged to cause closure of the contacts 5|a when it issubject to a voltage corresponding to a safe slow speed of the motor l0.Therefore, shortly after the switches l1 and III are closed to move thehoist out of the limit, the switch 5| closes its contacts Sla tocomplete the shunt circuit around the armature winding II. The rate ofacceleration and ulti mate speed of the motor Ill under such conditionsare greatly reduced, and the motor speed is at a very low value when thelimit switch resets so that there is very little, if any, change inspeed as the hoist mechanism comes out of the limit.

The winding Blw is apt to be energized and cause closure of the contacts5la during emergency dynamic braking. This, however, does not greatlyaffect the dynamic braking torque. It is to be noted that the switch 5|and the resistor 52 are connected for mounting on the moving part of thehoist so that it is not necessary to in-- crease the number of trolleybars.

A certain '75 ton crane driven by a 230 volt, 100 horsepower motor, onwhich the control system of this inventionwas subsequently installed,

formerly attained a speed of 900 R. P. M. on r the first lowering pointwhen moving out of the limit stop with an empty hook but when the limitswitch reset, the speed was suddenly reduced to R. P. M. causing severeflashing on the commutator and dangerous strains on the hoistingmechanism. The resistance of the resistor of this installation is 0.74ohm and the armature shunt resistor 52 having a resistance of 0.185 ohmwas added in accordance with the present invention. Contactor 5| alsowas added and arranged to close the circuit through the resistor 52 whenthe counter voltage of the motor reached volts. The motor speed with theadded equipment was 120 R. P. M. when lowering out of the limit stopwith an empty hook as compared with the former speed of 900 R. P. M. Inlowering a 30 ton load the lowering speed as a series motor with thepresent armature shunt was 160 R. P. M. which reduced to 62 R. P. M.when the limit switch reset. A crane which the purchaser refused to usebecause obviously the structure would be damaged and dangerous, wasequipped with the present invention and thereafter used for heavy loadsand found safe and efiicient.

The modification disclosed in Fig. 5 employs a limit switch similar tothe limit switch l0 but which has an extra normally-open contact 42carranged to close the armature shunt circuit through the resistor 52concurrently with the closing of the contacts 420 and 42d when the limitswitch trips. The same result is accomplished as is accomplished in thecontrol system of Fig. 1, but the extra contact 42c on the limit switchrequires a special construction.

Although operation of the contactor 5| by the voltage drop across themotor lll during emergency dynamic braking is of little consequence,such operation can be prevented by the insertion of a contact typerectifier 53 in series with the winding 5110. The voltage drop acrossthe resistor 50 is in one direction during emergency dynamic braking andin the other direction when the motor in is connected to lower out ofthe limit stop Ml, and, therefore, with the rectifier 53 arranged at thecorrect polarity, the contactor 5| closes during lowering with the limitswitch 10 tripped but does not close during emergency dynamic braking.

Although this invention has been described in connection with a dynamicbraking hoist controller of the type shown in my reissue Patent No.19,547, its operation in connection with other types of controllers isobvious and the invention is not to be limited to the exemplarystructures and uses illustrated.

Iclaim:

1. A control system for an electric motor arranged to operate a memberin two directions and having an armature winding and a field winding,which windings are adapted for shunt and series connection, selectively,with each other, for operation of the motor as a shunt and as a seriesmachine, selectively, and adapted for connection to a source of power,said system comprising a dynamic braking circuit for said motor, areversing controller for said motor, switch means responsive to movementof said member to a predetermined limit of travel in one direction fordisconnecting the motor from the source of power and for completing saiddynamic braking circuit for the motor, said reversing controllerincluding means to connect the motor to a source of power as a seriesmotor for moving said member in the opposite direction while saiddynamic braking circuit remains completed, a slow speed means operableto modify said last named connection to the source of power while thelast named connection is completed to change the excitation of the motorfor moving said member at a slow speed, and electroresponsive meansnormally disconnected from the motor and arranged to be electricallyconnected to the motor by said switch means and when connected to themotor being responsive to an electrical condition of the motor forcausing operation of said slow speed means only until said dynamicbraking circuit is interrupted by said switchmeans as a result of themovement of said member in the opposite direction.

2. A control system in accordance wtih claim 1 characterized in that theslow speed means includes means for completing an armature shuntconnection for said motor, whereby said change in the excitation of themotor is effected.

3. A control system in accordance with claim 1 characterized in that theelectroresponsive means comprises a contactor responsive to theelectrical condition of the motor and the slow speed means comprises anarmature shunt connection for said motor controlled by said contactor.

4. A control system for an electric motor arranged to operate a memberin two directions and having an armature windin and a field winding,which windings are adapted for shunt and series connection, selectively,with each other, for operation of the motor as a shunt and seriesmachine, selectively, and adapted for connection to a source of power,said system comprising a dynamic braking circuit for said motor, areversing controller for said motor, switch means responsive to movementof said member to a predetermined limit of travel in one direction fordisconnecting the motor from the source of power and for completingsaiddynamic braking circuit, said reversing controllerdrrcluding meansto connect said motor as a seriemmotor to a source of power for movingsaid member in the opposite direction while said dynamic braking circuitremains completed, said switch means being operable consequent uponmovement of said member in the opposite direction concurrently tointerrupt said series motor connection, to connect said motor as a shuntmotor to the source of power, and to interrupt said dynamic brakingcircuit, a slow speed means operable to provide a slow speed connectionfor said motor until said dynamic braking circuit is interrupted as aresult of the movement of said member in the opposite direction, anelectroresponsive means operable in response to an electrical conditionof the motor for permitting operation of said slow speed means.

5. A control system in accordance with claim 4 characterized in that theslow speed connection includes an armature shunt circuit having aresistance such that the speed of said motor, when the armature shuntcircuit is operative, is substantially the same as the speed oi. themotor after said dynamic braking circuit is interrupted.

6. A system of motor control for raising and lowering a load andemploying a direct current motor having an armature winding and a fieldwinding, which windings are adapted for shunt and series connection,selectively, with each other, for operation of the motor as a shunt andseries machine, selectively, and adapted for connection to a source ofpower, a dynamic braking resistor, means for connecting the motor tosaid source of power as a shunt motor for lowering a load, means forconnecting the motor to said source of power as a series motor forraising a load, switch means responsive to the elevated position of theload while a load is being raised for disconnecting the motor from saidsource and for connecting the motor in a dynamic braking circuit withsaid resistor, means for reconnecting the motor to said source as aseries motor for lowering the load while said dynamic braking circuitremains intact, said switch means being operable after the load has beenlowered a predetermined distance to interrupt said dynamic brakingcircuit and to reconnect the motor to said source of power as a shuntmotor, an armature shunt circuit including an armature shunt resistor,fixed electrical connections between said resistor and said armaturewinding, and contact means in said connections operable for completingsaid armature shunt circuit while said motor is connected as a seriesmotor and lowering the load.

7. A system in accordance with claim 6 characterized in that anelectroresponsive means connected to the motor and responsive to thecounter electromotive force of the motor effects operation of saidcontact means.

8. A system in accordance with claim 6 characterized in that the contactmeans is operated by operation of said switch means in response to theelevated position of the load during hoisting of said load.

9. In combination, an electric motor having a field winding and anarmature winding adapted to be connected in series and in parallel witheach other, selectively, limit switch means for disconnecting said motorfrom its source of supply at a predetermined limit of operation and forpartially completing reverse power connections between said motor andsaid source, directional switching means for completing said partiallycompleted connections, electroresponsive means normally disconnectedfrom said motor, said limit switch means including means operable toconnect said electroresponsive means to said motor upon disconnection ofsaid motor from its source by said limit switch means, and an armatureshunt circuit for said motor, said electroresponsive means beingresponsive to an electrical operating condition of said motor uponcompletion of said partially completed connections for connecting saidarmature shunt circuit to said motor.

10. In combination, an electric motor having an armature winding and afield winding and adapted to be connected to a source of power as ashunt and as a series motor, selectively, electroresponsive meansdisconnected from said motor while said motor is connected as a shuntmotor and when connected to said motor while said motor is connected asa series motor operable in response to an electrical condition of saidmotor for completing an armature shunt circuit for said motor, limitswitch means operable at a predetermined limit of operation of saidmotor for disconnecting said motor from said source and connecting saidelectroresponsive means to said motor, and directional switching meansfor reconnecting said motor to said source as a series motor with saidelectroresponsive means connected to said motor.

11. In a control system for an electricmotor arranged to operate amember in two directions and having an armature winding and a fieldwinding, which windings are adapted for shunt and series connection,selectively, with each other, for operation of the motor as a shunt andas a series machine, selectively, and adapted for connection to a sourceof power, the combination with a dynamic braking circuit for said motor,a reversing controller for said motor, switch means responsive tomovement of said member to a predetermined limit of travel in onedirection for disconnecting the motor from the source of power and forcompleting said dynamic braking circuit, said reversing controllerincluding means to connect the motor to the source of power for movingsaid member in the opposite direction while said dynamic braking circuitremains completed, and a slow speed means operable to modify said lastnamed connection to the source of power while the last named connectionis completed to change the excitation of the motor for moving saidmember at a slow speed, of electroresponsive means connected to themotor through said switch means and responsive to the countervoltage ofthe motor for causing operation of said slow speed means.

12. A control system for an electric motor having a winding adapted tobe connected as a series field winding and comprising, in combination, adynamic braking resistor, an electroresponsive means having an operatingcoil normally disconnected from said motor; limit switch mechanismoperable at a predetermined limit of operation in one direction ofrotation for disconnecting said motor from its source, for connectingsaid coil in parallel with said motor, and for completing a seriesdynamic braking circuit through said resistor, said field winding, andthe armature of said motor; directional switching means for completingreverse power connections from a point in the circuit between thearmature and said resistor to the source so that said motor is connectedto the source with said dynamic braking resistor in parallel with saidarmature and series field winding; said electroresponsive means beingresponsive to the terminal voltage of said motor upon completion of saidreverse power connections for completing an armature shunt circuit inparallel with the armature of said motor and being responsive to theoperation of said limit switch at said predetermined limit in thereverse direction of rotation of said motor for interrupting saidarmature shunt circuit.

13. In combination, an electric motor having a field winding, limitswitch means operable concurrently at a predetermined limit of motoroperation to disconnect said motor from its source, to connect saidmotor in a dynamic braking circuit including a resistor, and topartially complete reverse power connections from said motor to saidsource, directional switching means for completing said reverse powerconnections, an armature shunt circuit for said motor, anelectroresponsive means, and fixed connections completing a parallelcircuit including said dynamic braking resistor and saidelectroresponsive means, said electroresponsive means being responsiveto an electrical operating condition of said motor upon completion ofsaid reverse power connections for connecting said armature shuntcircuit to said motor.

14. A control system for an electric motor having a field winding and anarmature winding, an armature shunt circuit having terminals adapted tobe connected across said armature winding, an electromagnetic switchhaving an operating winding adapted to be connected across said mo torand having contacts in said armature shunt circuit, and limit switchmechanism operable at a predetermined limit of motor operation fordisconnecting said motor from its source and connecting said operatingwinding across said motor, said electromagnetic switch being responsiveto a voltage of said motor while said operating winding is connectedacross said motor for closing said contacts in said armature shuntcircult.

15. In combination, an electric motor having a field winding, anarmature shunt circuit for said motor, an electromagnetic switch havingan operating winding adapted to be connected to said motor and havingcontacts in said armature shunt circuit, which, when closed, completesaid armature shunt circuit, and limit switch means for disconnectingsaid motor from its source and connecting said operating winding to saidmotor at a predetermined limit oif operation, said switch means beingresponsive to a voltage of said motor while said winding is connected tosaid motor for closing said contacts and completing said armature shuntcircuit.

16. In a hoist control system and hoist motor combination, a hoist motorhaving a field winding adapted to be connected as a series and as ashunt winding. selectively, limit switch mechanism for disconnectingsaid motor from its source at a predetermined limit of operation and forpreparing reverse power connections from said motor to said source,directional switching means for completing said reverse powerconnections, an armature shunt resistor having one terminal fixedlyconnected to said motor, and means responsive to an electrical operatingcondition of said motor upon completion of said reverse powerconnections for completing an armature shunt circuit for said motorincluding said armature shunt resistor.

17. In a motor and control system combination, a plurality of trolleybar mechanism; a series circuit including a pair of limit switchcontacts, a motor armature winding, and a motor field winding connectedbetween one pair of said trolley bar mechanisms; control means for saidmotor connected between said trolley bar mechanisms and a source ofpower for the motor; and an armature shunt circuit for said motor and adynamic braking circuit for said motor each connected to said motorbetween said trolley bar mechanisms and said motor.

18. In a series type hoist motor and control system combination, aplurality of trolley bar mechanisms, control means for said motorconnected between said trolley bar mechanisms and a source of power forthe motor, a dynamic braking circuit for said motor arranged to becompleted by a limit switch, and an armature shunt circuit for saidmotor arranged to be completed by an electroresponsive means, saiddynamic braking circuit and said armature shunt circuit being fixedlyconnected to said motor inside of said trolley bar mechanisms.

19. A hoist motor and hoist control system combination comprising afirst group of electrical devices with fixed electrical connectionstherebetween and including a plurality of switches and a plurality ofresistors for controlling the operation of said hoist motor; a secondgroup of electrical devices with fixed electrical connectionstherebetween and includin said hoist motor, a limit switch means, adynamic braking circuit controlled by said limit switch means, anarmature shunt circuit, and a switch having an operating meansresponsive to a voltage of said motor and operable to control aconnection between said armature shunt circuit and said motor; andelectrical connections between said groups of devices enabling relativemovement of said groups.

20. A hoist motor and hoist control system combination comprising aplurality of trolley bar mechanisms; a first group of electrical deviceswith fixed electrical connectionsv therebetween and including aplurality of switches and a plurality of resistors for controlling theoperation of said hoist motor; a second group of electrical devices withfixed electrical connections therebetween and including said hoistmotor, a limit switch means, a dynamic braking circuit controlled bysaid limit switch means, an armature shunt circuit, and a switch meanshaving an operating means responsive to a voltage of said motor forcontrolling a connection between said armature shunt circuit and saidmotor; and a plurality of electrical connections between said groups ofdevices including said trolley bar mechanisms, respectively, wherebyrelative movement of said groups is enabled while said electricalconnections between said groups are maintained.

21. In combination, an electric motor having a field winding, arectifier, electroresponsive means responsive to an electrical conditionof said motor for completing an armature shunt circuit for said motor,and limit switch mechanism for disconnecting said motor from its sourceand connecting said electroresponsive means to said motor in series withsaid rectifier at a predetermined limit of operation.

22. A control system for hoists in which the hoist is driven by anelectric motor having a field winding comprising in combination, anelectromagnetic switching device for controlling the continuity of anarmature shunt circuit for said motor and having an operating coiladapted to be connected to said motor so as to be responsive to anelectrical condition of said motor, a rectitier, an overhoist limitswitch mechanism for disconnecting said motor from its source at apredetermined limit or operation or said motor in one direction, forconnecting said operating coil to said motor in series with saidrectifier to provide for completion oi said armature shunt circuit inresponse to a predetermined electrical condition of said motor, and fordeenergizing said coil at said limit or operation in the reversedirection of rotation of said motor, thereby to interrupt said armatureshunt circuit.

23. In a system or motor control for raising and lowering a load andemploying a direct current motor having an armature winding and a fieldwinding, which windings are adapted for shunt and series connection,selectively, with each other, for operation 01' the motor as a shunt andseries machine. selectively, and adapted for connection to a source ofpower, and a dynamic braking circuit for the motor, the combination withmeans for connecting the motor to the source of power as a shunt motorfor lowering a load, means for connecting the motor .to said source ofpower as a series motor for raising the load, switch means responsive tothe elevated position of the load while a load is being raised fordisconnecting the motor from said source and for connecting said motorin a dynamic braking circuit, and means for reconnecting the motor tosaid source as a series motor for lowering the load while said dynamicbraking circuit remains intact, said switch means being operable afterthe load has been lowered a predetermined distance to interrupt saiddynamic braking circuit and to reconnect the motor to said source ofpower as a shunt motor, 01' an armature shunt connection for the motorand operable to slow down the motor, when the motor is operating as aseries motor, to provide a motor speed approximately equal to the normalshunt motor lowering speed, and said switch means being operable forrendering said armature shunt connection efiective while said load isbeing lowered said predetermined distance, whereby the motor speed doesnot change appreciably when said switch means operates to reconnect themotor as a shunt motor.

24. A control system for a dynamo electric machine having an armaturewinding and a field winding and adapted to be connected as a series andas a shunt machine, selectively, comprising in combination, a dynamicbraking resistor, an armature shunt resistor, limit switch means fordisconnecting said machine from its source at a predetermined limit oroperation in one direction of rotation and completing a series dynamicbraking circuit through said dynamic braking resistor. the fieldwinding, and the armature winding of said machine, directional switchingmeans for completing reverse power connections to said source from apoint in the dynamic braking circuit between said armature winding andsaid dynamic braking resistor so that said machine is connected to saidsource with said dynamic braking resistor in parallel with a seriesconnection including the armature winding and the series field winding,said limit switch means including means operable upon completion of saidseries dynamic braking circuit for connecting said armature shuntresistor in parallel with the armature winding of said machine, and saidlimit switch means :being operable at said predetermined limit in thereverse direction of rotation oi. said motor for interrupting saidarmature shunt circuit and completing power connections irom saidmachine to said source with said field winding in a parallel circuitwith said armature winding.

DAVID C. WRIGHT.

